NAME

       cc_cdg — CDG Congestion Control Algorithm

DESCRIPTION

       CAIA-Delay  Gradient  (CDG) is a hybrid congestion control algorithm which reacts to both packet loss and
       inferred queuing delay.  It attempts to operate as a delay-based algorithm where possible,  but  utilises
       heuristics  to  detect  loss-based  TCP  cross  traffic and will compete effectively as required.  CDG is
       therefore incrementally deployable and suitable for use on shared networks.

       During delay-based operation, CDG uses a delay-gradient based probabilistic backoff mechanism,  and  will
       also  try  to  infer  non congestion related packet losses and avoid backing off when they occur.  During
       loss-based operation, CDG essentially reverts to cc_newreno(4)-like behaviour.

       CDG switches to loss-based operation when it detects that a configurable  number  of  consecutive  delay-
       based  backoffs  have  had  no  measurable  effect.   It  periodically  attempts to return to delay-based
       operation, but will keep switching back to loss-based operation as required.

MIB Variables

       The algorithm exposes the following variables in the net.inet.tcp.cc.cdg branch of the sysctl(3) MIB:

       version            Current algorithm/implementation version number.

       beta_delay         Delay-based window decrease factor as a percentage (on delay-based backoff, w  =  w  *
                          beta_delay / 100).  Default is 70.

       beta_loss          Loss-based  window  decrease  factor  as  a percentage (on loss-based backoff, w = w *
                          beta_loss / 100).  Default is 50.

       exp_backoff_scale  Scaling parameter for the probabilistic exponential backoff.  Default is 2.

       smoothing_factor   Number of samples used for moving average smoothing (0 means no  smoothing).   Default
                          is 8.

       loss_compete_consec_cong
                          Number  of  consecutive  delay-gradient  based  congestion episodes which will trigger
                          loss-based CC compatibility.  Default is 5.

       loss_compete_hold_backoff
                          Number of consecutive delay-gradient based congestion  episodes  to  hold  the  window
                          backoff for loss-based CC compatibility.  Default is 5.

       alpha_inc          If  non-zero,  this  enables  an  experimental mode where CDG's window increase factor
                          (alpha) is increased by 1 MSS every alpha_inc RTTs during congestion  avoidance  mode.
                          (Setting  alpha_inc  to 1 results in the most aggressive growth of the window increase
                          factor over time.  Use higher alpha_inc values for slower growth.)  Default is 0.

SEE ALSO

       cc_chd(4), cc_cubic(4), cc_hd(4), cc_htcp(4), cc_newreno(4), cc_vegas(4), h_ertt(4),  mod_cc(4),  tcp(4),
       khelp(9), mod_cc(9)

       D.  A.  Hayes and G. Armitage, “Revisiting TCP Congestion Control using Delay Gradients”, Networking 2011
       Proceedings, Part II, 328-341, May 2011.

       N. Khademi and G. Armitage, Minimising RTT across homogeneous 802.11 WLANs with CAIA  Delay-Gradient  TCP
       (v0.1),   CAIA  Technical  Report  121113A,  http://caia.swin.edu.au/reports/121113A/CAIA-TR-121113A.pdf,
       November 2012.

ACKNOWLEDGEMENTS

       Development and testing of this software were made possible in part by grants from the FreeBSD Foundation
       and The Cisco University Research Program Fund, a corporate advised  fund  of  Silicon  Valley  Community
       Foundation.

HISTORY

       The cc_cdg congestion control module first appeared in FreeBSD 9.2.

       The  module  was  first  released in 2011 by David Hayes whilst working on the NewTCP research project at
       Swinburne University of Technology's Centre for Advanced Internet  Architectures,  Melbourne,  Australia.
       More details are available at:

       http://caia.swin.edu.au/urp/newtcp/

AUTHORS

       The cc_cdg congestion control module was written by David Hayes <david.hayes@ieee.org>.  This manual page
       was written by Lawrence Stewart <lstewart@FreeBSD.org> and Grenville Armitage <garmitage@swin.edu.au>.

BUGS

       The  underlying  algorithm  and  parameter values are still a work in progress and may not be optimal for
       some network scenarios.

Debian                                            July 2, 2013                                         CC_CDG(4)